1. Field of the Invention
The present invention relates to an arrangement for anchoring a rod-shag tension member of composite fiber material which is composed of unidirectional fibers embedded in a matrix of synthetic resin, particularly for use as an anchoring element in the construction of foundations, in underground mining, as a reinforcing element for concrete, particularly for pre-stressed concrete, and the like.
2. Description of the Related Art
In recent years, instead of tension members of steel, tension members of high-strength composite fiber materials have increasingly been used in structural engineering projects. While having approximately the same tensile strength as tension members of steel, tension members of composite fiber materials, such as polyamid fibers, aramid fibers, glass fibers, or carbon fibers, have a lower specific gravity as compared to those of steel, and they particularly have a greater resistance to corrosion. Accordingly, because of these properties, tension members of composite fiber materials can be used advantageously as anchoring elements in foundation work and underground mining, where aggressive water frequently occurs which poses the danger of corrosion to tension members of steel.
Moreover, the behavior of tension members of composite fiber materials, when used under load, is similar to that of tension members of steel, i.e., they have an essentially linear stress-strain line. However, contrary to tension members of steel, tension members of composite fiber materials do not have a yield point; rather, they break when the breaking strength or tensile strength of the tension member is reached without prior plastic deformation. Because of this property and because of the fact that, in contrast to the tensile strength in longitudinal direction of the fibers, the tension members of composite fiber materials only have a low transverse compressive strength, anchoring arrangements which are otherwise known and proven in structural engineering cannot be directly transferred to anchoring arrangements which use tension members of composite fiber materials.
The use of threaded anchoring arrangements with conventional thread profiles poses a problem when anchoring tension members of composite fiber materials because the tensile force in the tension member is transmitted in the region of the threads to the respective anchoring element essentially through an axial shearing force which causes shearing stress in the threads. In order to prevent the threads from shearing off, to prevent the fiber strand from breaking open in radial planes, and to prevent cylindrical outer portions from shearing off relative to cylindrical inner portions in the region of the thread of rod-shaped tension members, it is known in the an to arrange the fibers in an outer region in alternating inclined directions and to anchor the outer fibers by means of textile interlinking with fibers which are located deeper inside (DE OS 35 04 829). However, even in a thread produced in this manner, the shearing effect remains dominant when a load is applied.
In an anchoring arrangement of the above-described type, it is also known in the art to arrange a buffer layer as a plastification zone between a tension member of composite fiber material and an anchoring member of steel, and particularly between the sides of the threads of these two elements which are in engagement with each other (DE OS 38 34 266). The strength of the material used for this buffer layer must be lower than the strength of the composite fiber material with respect to transverse compression, so that, under load and after a certain compressive stress has been reached, substantial deformations are possible with small increases of stress.
In this connection, it has also been proposed to manufacture the profiles of the tension member, which may be composed of an asymmetrical trapezoidal thread, by applying a deformation pressure on the rod from the outside in radial direction during hardening of the synthetic resin which surrounds and glues together the unidirectional fibers. The deformation pressure impresses thread valleys into the rod. Consequently, the fibers of the rod are not severed; rather, the direction of the fibers is only slightly deflected.
Finally, it is also known in the art to produce an anchoring element as a nut of plastics material in which the threads are reinforced by a reinforcement composed of warps and wefts (EP 0 403 940 A1). However, this type of reinforcement also does not reduce the shearing stresses in the threads of the tension rod.